1. Field of the Invention
The present invention relates to a servo unit of a braking system for use in an automotive vehicle, and more particularly to a hydraulic brake booster which actuates a master cylinder by a hydraulic pressure supplied from a power source in response to actuation or depression of a brake pedal.
2. Description of the Prior Art
In a conventional service braking system for an automotive vehicle, a servo unit is provided which may be alternatively referred to as a booster. The unit permits a reduction in the force required to operate or actuate a brake pedal in a braking operation. The servo unit utilizes compressed air, intake manifold vacuum, or hydraulic pressure as a power source and are distinguished from an air brake booster, a vacuum brake booster, or a hydraulic brake booster operated in response to a power source. The hydraulic brake booster includes a hydraulic pressure control valve actuated in response to the actuation or depression of the brake pedal and which regulates the hydraulic pressure supplied from the power source to a boost pressure and actuates the master cylinder by the boost pressure. In a known hydraulic brake booster, a lever operated type controls the hydraulic pressure control valve by a pair of levers which consists of a support lever and a control lever actuated by the operation of the brake pedal and the operation of a power piston actuated in response to the boost pressure, respectively. A conventional hydraulic brake booster of this kind is disclosed, for example, in Japanese application Laid-open publication No. 50-74079 published on June 18, 1975. In this conventional hydraulic brake booster, the hydraulic pressure control valve is constituted by a spool valve. In operation of this conventional hydraulic brake booster, only a small shock is generated when the hydraulic control valve is opened and shut, thus permitting smooth operation.
In the above prior hydraulic brake booster, however, since the pair of levers which control the operation of the spool valve are supported or hung by a pin fixed to a housing, a positional relationship between the levers and the spool valve is rigidly defined. Accordingly, the accuracy of the dimensions of the spool, levers, housing and so on are very important when lap quantity of the spool valve is determined. Further, the limits of the dimensional accuracy requires a design which takes into consideration an error of measurement which exceeds the limits of accuracy and needed scattering. Particularly, the lap quantity is an important factor which is of concern in an idle stroke when the spool advances, ensures the fluid flow when the spool retreats, provides for a reduction of leakage and so on. In the hydraulic brake booster of the prior art, an opening lap of the drain port is determined largely due to the importance of providing fluid flow when the spool retreats. As a result, the prior hydraulic brake booster is compelled to increase the idle stroke when the spool advances.